This invention relates to a magnetic recording medium, and more particularly, to a magnetic recording medium capable of achieving high-density recording and having good mechanical properties.
Magnetic recording is a technique that achieves high-density magnetic recording, and repeated recording and reproduction is possible with this technique. Because of this, the magnetic recording technique is employed in recording many kinds of signals, including audio signals, video signals, and digital signals. In view of the desirable features of magnetic recording, sales of audio tape and video tape for miniaturized video tape recorders (VTRs) for for home- and office-use have greatly expanded, and their quality has also improved greatly.
To record video signals at a rate of 30 frames per second, which is the standard signal rate for television, a very large signal unit of from 1 to 10 MHz per second must be recorded in the form of a difference in magnetization on magnetic tape. One of the most important targets in efforts to improve the quality of magnetic tape is to make the minimum recording unit on magnetic tape as small as possible; in other words to get the highest possible recording density per unit length on the magnetic tape because this directly enables the use of small-sized magnetic tape, and hence miniaturized video taper recorder.
In recording with a ring-shaped magnetic head of the type most commonly used in magnetic recording, the distribution of magnetizing force in front of the magnetic head gap decreases generally in proportion to the distance from the head face. Accordingly, the larger the distance from the head face, the smaller the magnetizing force acting on the magnetic tape.
If .lambda. is the recording wavelength (i.e., the head to magnetic tape speed divided by the frequency of the signal recorded), d is the distance between a certain point in the magnetic layer of a magnetic tape and the surface of that layer, and a is the gap between the head face and the surface of the magnetic layer of the magnetic tape, the output of the signal reproduced decreases at a rate of 55.5 (d+a)/(.lambda.) decibels (dB). Therefore, if the recording wavelength is small, the reproduction output decreases rapidly even if the value of d+a is small. If the recording wavelength .lambda. becomes less than 1 to 2.mu. and less than the thickness of the magnetic layer d.sub.o (which is generally 3 to 12.mu.), magnetization in the part of the magnetic tape remote from the head does not effectively contribute to reproduction output. When .lambda. is greater than 10 to 100.mu. and is adequately larger than the thickness of the magnetic layer d.sub.o, the magnetization of every part of the magnetic layer contributes to the reproduction output.
A method has been developed to provide video tape or other magnetic tape that is capable of high-density recording, comprising increasing the coercive force of the magnetic layer to a level high enough to overcome the demagnetizing field created by self-demagnetization from high recording density. Chrome dioxide, Co-modified iron oxide, and finely divided iron alloys are examples being used commercially. The self-demagnetizing effect can be reduced by decreasing the thickness of the magnetic layer for a given recording density on the magnetic tape, and on the basis of this idea, plating or vapor deposition of Co alloy have been proposed as a technique for high density magnetic recording.
These techniques primarily contribute to the output produced in high recording density for d.sub.o /.lambda..gtoreq.1. With video tape for use in VHS (Video Home System, a video recording/reproducing system of Japan Victor Co.) or Beta-format system (a video recording/reproducing system of Sony Co.), color signals and audio signals are recorded in relatively low density (d.sub.o /.lambda.&gt;&gt;1). To provide adequate reproduction output in such low recording density, a magnetic layer as thick as from about 3 to 6.mu. is used. The strength of the recording magnetic field decreases rapidly as the distance from the surface of the recording magnetic head increases. This strength of the recording magnetic field decreases generally in proportion to (d+a)/g where "g" is gap width of the head. Typical gap lengthes are as follows. The head gap width g is from 4 to 1.5.mu. for audio recording head, from 2 to 1.mu. for an audio record/reproduce head, and from 1 to 0.4.mu. for video head. Therefore, many methods have been proposed to provide high reproduction output in low as well as high recording density (1&lt;&lt;&lt;d.sub.o /.lambda.-d.sub.o /.lambda.&gt;&gt;1) for a particularly small head gap (d.sub.o /g&gt;1) by decreasing the coercive force continuously or stepwise from the surface to the bottom of the magnetic layer. Such methods have been described, for example, in U.S. Pat. Nos. 2,691,072, 2,643,130, 2,647,954, and 3,761,311. An arrangement wherein a magnetic layer made of an iron alloy having a relatively large coercive force is formed on a magnetic layer made of iron oxide (having a relatively small coercive force) is also proposed in Japanese Patent Publication No. 2218/62.
However, because of the size of the magnetic particles used and the application technique available, it is very difficult to coat these prior art magnetic recording media with a thin (less than 0.5.mu.) uniform layer of overcoating wherein an iron alloy powder having high coercive force and capable of producing high output in high recording density is dispersed in a binder. In addition, in view of the recent trend of using a small head gap, e.g., as small as from 2 to 0.3.mu., an even thinner topcoat is required, and this makes it even more difficult to achieve high recording density and provide adequate reproduction output by the above prior art.